Find top-rated Pros in your area

Enter a zip code and get matched to businesses near you.

Deck Doctor Llc

Decks and Porches, Deck Maintenance, Fencing

About us

We have been dedicated to deck refinishing and deck repair in Denver for almost 20 years. Deck Doctor is also the largest and oldest company in Colorado that is dedicated to deck refinishing and repair. We have refinished 1000’s of decks since 1993 and guarantee that you will be completely satisfied with our service and professionalism. Deck Doctor understands how important your home and deck are to you. That's why we go the extra mile when preserving and refinishing your investment. With Deck Doctor you can rest assured that your deck, fence, gazebo, or furniture will be properly stained and refinished to preserve its integrity for many summers to come. Our work is done by employees (not contractors) with workman compensation insurance. In addition, we carry a million dollar liability insurance policy. Be sure to check these important requirements with all deck refinishing contractors you contact.

Business highlights

31 years of trusted experience

Services we offer

Deck sanding, deck or redwood sealant application, power washing, deck repair, fence refinishing & composite work.

Amenities

Free Estimates
Yes


Accepted payment methods

Check
MasterCard
Visa

Reviews

4.120 Reviews
Number of StarsImage of DistributionNumber of Ratings
5
65%
4
0%
3
10%
2
5%
1
20%


Rating CategoryRating out of 5
quality
4.2
value
4.0
professionalism
4.0
responsiveness
4.2
punctuality
4.2
Showing 1-20 of 20 reviews

Dawn M.
07/2024
3.0
decks
  + -1 more
Replaced our front deck last spring. In comparison to what it looked like originally, it is way better. Took twice as long as they estimated and the price creeped up incrementally, but hey, typical contractor. Now for the bad part...one of the boards they used was notched in multiple places leaving a splintered edge for any barefoot walker to injure their tender toes. Another board started peeling almost immediately and at best needs sanding and refinishing. At worst replacing. I called and they told me they had no way of knowing that the boards were bad when they installed them. Somehow that didn't help with the fact that the boards are indeed bad. I complained further to the actual owner. He said they'd come back and fix it this past spring. Guess what I got? Silence. As the other reviews suggest, try someone else if you want quality service and/or someone who knows good wood vs. bad.
Description of Work: Deck replacement

Rating CategoryRating out of 5
quality
2.0
value
3.0
professionalism
2.0
responsiveness
2.0
punctuality
3.0

$18,000

Sarah A.
10/2022
5.0
decks
  + -1 more
Brandon and his team are amazing! They do what they say, when they say, and the quality of work is top-notch. My deck needed a complete overhaul due to rotting wood and poor infrastructure that was also not to code. Brandon worked with me on the budget that I had, offering his expertise and recommendations as to what "absolutely" needed to be done vs. what could wait until Spring. His crew is honest and hard working (who works until 6pm on a Friday anymore!) - both of which are very tough to find. Last but not least, at the end of each work day the crew cleaned up, swept, and hauled away materials. I didn't have to worry about nails, wood, and whatever else all over my driveway at the end of each day, which is also important with young kids. If you are looking for deck repair or a complete deck overhaul, I highly recommend Deck Doctor! Brandon and his team with 25+ years of experience are genuinely good people!
Description of Work: Rotting wood, Sagging frame, paint and repair

Rating CategoryRating out of 5
quality
5.0
value
5.0
professionalism
5.0
responsiveness
5.0
punctuality
5.0

Yes, I recommend this pro
$48,000

Renee B.
06/2022
1.0
decks
  + -1 more
See attached pictures, explains the issues. The owner threatened to lien my property.
Description of Work: Deck Repair

Rating CategoryRating out of 5
quality
2.0
value
2.0
professionalism
1.0
responsiveness
1.0
punctuality
1.0


Response from Deck Doctor Llc
Mrs. [Member Name Removed] had some concerns we were happy to address and take care of. During my inspection and conversation with her, she became very upset, yelling and demanding, she then would not communicate or pay for the project. I communicated through her email apologizing, adding additional compromises, and asking for her cooperation and communication, but no response, until this complaint. I explained what I would be forced to do if she didn't pay for the work completed with the discount I offered her, which includes me filing a mechanics lien. She paid us by deducting double the discount I offered. I decided to move on without any filings or court.

Kim T.
06/2019
5.0
decks
  + -1 more
Job involved rebuilding my deck because I wanted to install Trex which is much heavier than wood, requiring 12 in. joist spacing vs. 16 in. I also wanted to use my existing metal railing, so Deck Doctor (DD) workers had to build the new deck with exact measurements to the old deck which also included an odd angle on one side. DD workers also had to take down a heavy scaffolding type structure that I had on one side of the deck to hold a sunshade, and then put it back up again in a precise location so it would bump against my deck rollout shade. They poured concrete footings and a concrete pad at the bottom of the stairs which I didn't have before. They also attached a lattice skirt (which I bought and stained). I love the new deck and think they did a great job. Justin, the sales rep, was great to work with, answered all my questions, and gave assurances that they would make sure I was happy with the final product. However, Justin is very busy, so it's helpful to have a bit of patience and a willingness to text. There were a couple of surprises which required more work and more material expense due in part to my change of mind and in part to having to construct the stair support from scratch which wasn't anticipated. This increased the cost so just be aware that the bids can't always anticipate everything. Still, I think the price was reasonable given that Trex is very expensive and was a significant component of the total cost. The DD workers were very respectful, cleaned up every day and kept track of all the details of constructing the deck identical to the old one. My deck dimensions are 18' by about 13'. I would hire Deck Doctor again.
Description of Work: Tear down existing deck and rebuild in identical size

Rating CategoryRating out of 5
quality
5.0
value
5.0
professionalism
5.0
responsiveness
5.0
punctuality
5.0

Yes, I recommend this pro
$9,500

Daniel M.
08/2017
5.0
decks, deck cleaning
  + 0 more
Brandon's crew did a tremendous job of shoring up our older deck (21 year old). They removed and replaced several joists, removed an old staircase from the upper deck to the mid deck and build a replacement railing on the upper deck. Lastly the crew removed a portion of the lower deck and installed new foundations and a four supporting structure to support the weight of the hot tub. The work crew was amazing to watch as they were very professional, safety conscious and their work was top shelf. The even did a great job of cleaning up post work. I could not be more pleased with the results of the work.
Description of Work: The deck on the back of our home was in need of repair and refurbishment. We also purchased a hot tub and the lower deck had to be rebuild to handle the weight of the new tub.

Rating CategoryRating out of 5
quality
5.0
value
4.0
professionalism
5.0
responsiveness
5.0
punctuality
5.0

Yes, I recommend this pro

BRADFORD M.
02/2014
5.0
deck cleaning
  + -1 more
.
Description of Work: .

Rating CategoryRating out of 5
quality
5.0
value
5.0
professionalism
5.0
responsiveness
5.0
punctuality
5.0

Yes, I recommend this pro

Alison S.
10/2013
1.0
decks
  + -1 more
I called for an estimate. I was told they were very busy but the owner would contact me when he arrived to the office that day. It's been over a week and I have not had my call returned.
Description of Work: I called for an estimate. I was told they were very busy but the owner would contact me when he arrived to the office that day. It's been over a week and I have not had my call returned.

Rating CategoryRating out of 5
responsiveness
1.0


Nancy B.
08/2013
5.0
deck cleaning
  + -1 more
They did a great job, were professional, friendly, I enjoyed working with them and they were responsive.
Description of Work: They re-stained my deck.

Rating CategoryRating out of 5
quality
5.0
value
4.0
professionalism
5.0
responsiveness
5.0
punctuality
5.0

Yes, I recommend this pro
$700

Darrell C.
08/2013
1.0
deck cleaning, decks
  + 0 more
Not well. I've contacted them 3 times to get my deck refinished this year. They finished my deck last year, but I cannot get them to come and perform an estimate or perform the work.
Description of Work: Nothing

Rating CategoryRating out of 5
professionalism
1.0
responsiveness
1.0
punctuality
1.0

$800

BRADFORD M.
05/2013
5.0
decks
  + -1 more
Excellent work.very satisfied.
Description of Work: Stripped , sanded and refinished deck.

Rating CategoryRating out of 5
quality
5.0
value
4.0
professionalism
5.0
responsiveness
5.0
punctuality
5.0

Yes, I recommend this pro
$1,600

Patty B.
05/2013
5.0
deck cleaning, decks
  + 0 more
Quality, Quality, Quality!!! Hard to believe since I am the picky women that insists that everything is done perfect, I found that his standards were higher than mine. I came to trust his judgement. If he saw work that was not being done up to his personal standard, he had the worker redo the work. He always returned calls and was in constant contact with the crew when he could not be on the job site. I NEVER felt like I was not his most important job. Saftey, pride in workmenship and clean-up were the standard not the exception. He was HONEST!!! I had six contractors from angies list come and give me quotes for the job. Several could not repair the decks, but woud GLADLY sell me a new decks for the price of my daughters college education. Others quoted me so low that it was clear that when they got into the job and "discovered more rot" , that the price would greatly increase with unforeseen costs. Others would only do part of the repairs and I would have to hire a seperate contractor for the rest of the job. He was honest with how much work my deck really needed!! No homeowner wants the bad news. It was clear that our safety was important to him, rather I gave him my bussiness or not. He was going to make sure I was awear of the extent of the rot. He spent severall hours with me so I could clearly understand the issues and show them to my husband when he got home. For example, I had a stairway that was barely standing. We hardly ever used it so I had no idea that it was so shakey. Not one other contractor pointed out the issue. Understanding of homeowner financial limitations. He took the time to talk about the problems and gave me several possible solutions. He took the time to give me three quotes. Giving me options, I never felt pressured or just being cheep. It was never all or nothing with him. Many of the other contractors were not very flexable. The one thing he would not be flexable on was quality - the job would be done to the highest standards or not done at all. His flexability allowed me to have a trex floor on my back deck yet use the wood railing. Without this option, I could not afforded trex on the back deck. Not one other contractor even offered me this option.
Description of Work: Had layers of paint removed/re-stained on three decks and my back deck had to be rebuilt due to rot. The removal and restain included a large wrap around deck, back deck railings that go the entire length of the house and a side deck. They did an excellent job!!!! They had to take down the railings to sand each rail and before they re-stained the deck it looked like a brand new deck. There was no sign of old paint before they began to stain. This was an amazing feat. The rot on the back deck was so bad, that he basically had to build a new deck. He installed a trex deck for the floor and we reused the wood railings since they were in great shape and this made the project more affordable. He really is the Deck Doctor, he took an old house that has decks compleately surrounding three sides of the house that were covered with years of built up cracking paint and rot and compleately changed the look back to beautiful wood. My back deck is rot free with a trex floor. The Doctor saved the deck and I am truly greatful. I now have a deck guy for life. Any future refinishing or repair the Deck Doctor will be my only call!!!!!!

Rating CategoryRating out of 5
quality
5.0
value
5.0
professionalism
5.0
responsiveness
5.0
punctuality
5.0

Yes, I recommend this pro
$13,000

Darrell C.
02/2013
5.0
deck cleaning, decks, fencing
  + 1 more
They were very good. Well, I think they are pretty thorough though they didn't sand the stain in the lower parts because you can still see the old stain there. I didn't notice when they're still here but later on saw that, but they weren't bad. It looks pretty good. I thought they were good professionally. They have a fair price.
Description of Work: I got a quote from Deck Doctor LLC and they refurnished my deck.

Rating CategoryRating out of 5
quality
4.0
value
4.0
professionalism
5.0
responsiveness
5.0
punctuality
5.0

Yes, I recommend this pro

Monica C.
01/2013
2.0
decks
  + -1 more
Bad. Will not ever call them again.
Description of Work: The man who came and gave us the quote didn't know what he was talking about and we never worked with him again. He told us what color to use and it didn't look at all like what he said. The supervisor of the crew came for a few minutes of the work just to meet us. The crew left screws and rags all over our yard. They left our table, chairs and grill down on our grass in the yard instead of bringing them back up. The worst was two huge stains left on our flagstone patio. I told them they needed to clean those before I would pay and the crew came without telling me and power washed the entire flagstone patio (leaving an even bigger mess). The stains were still there so the owner came out and apologized, but in the end said he could only take off $50 from the price. I still have my patio table down in the yard and huge stains on my flagstone patio. They were the most expensive quote with some good reviews so we used them. I will never call them again.

Rating CategoryRating out of 5
quality
3.0
value
2.0
professionalism
2.0
responsiveness
3.0
punctuality
2.0

$2,000

Response from Deck Doctor Llc
I represent Deck Doctor and I'm writing this in response to (member name removed) concerns to the work provided. We strive to provide the best quality at a fair price for deck refinishing, repair and replacement. Our training for estimators, supervisors and work technicians is focused in our off season but is also continued through the work season. We review each job and use unforeseen or unfortunate circumstances to help provide better service in the future. This job was supervised during work and revisited after the concerns were noted, by the work supervisor. Color choices always have a degree of unpredictability on aged wood surfaces, that is why the final color choice is the customers. Although masking and tarps are used, unfortunate mistakes happen and an attempt to remove them with the experience and proven materials is what we did. The method used of disassembling the total railing to sand it, although practice is made to retain old screw, sometimes leaves a few screws behind. Removing deck furniture is always done by us at no charge, replacement of deck furniture and bbq's is also done at no charge but requires requested assistance from our customers. A misunderstanding was experienced here and was taken care of by us after we learned what was needed. The managers visit to meet the (member name removed) was pleasant and completed with a discount to help offset any cost that might be experienced by an attempt to remove a small stain from a piece of sand stone. I apologize again for the bad feelings left from our attempts to make a unpleasant situation better. The before and after pictures confirm a dramatic improvement to the decks overall condition and help to preserve it's useful life. Thank you for your interest.

Darrell C.
09/2012
5.0
decks
  + -1 more
Good
Description of Work: Sand and refinish decks

Rating CategoryRating out of 5
quality
4.0
value
5.0
professionalism
5.0
responsiveness
5.0
punctuality
4.0

Yes, I recommend this pro
$2,500

Laura W.
09/2012
1.0
decks
  + -1 more
This was a long distance hire since I live in California and the job was in the Denver area. My tenants received the estimate on replacing the entire deck (second story condo) versus repairing. We chose repair since the HoA was not going to reimburse us for a replacement nor a repair. We wanted the deck railing repaired as needed, sanded, and painted and the deck floor boards replaced as needed, sanded, and stained/sealed. The company even came back out for a paint sample to match what was there already. First, they mentioned something about coming out on Aug. 20. That didn't happen. Then they made an appointment to come out on August 23 - no show. I was visiting and waited all day with no call after I specifically asked that I be the point of contact that day since I was paying the bill. The excuse was it was raining at their office and another job site all day - it didn't rain at all where we were at and they didn't call to find out. Then they rescheduled for August 25 - again, no show - excuse: it took longer at another job site than they expected. They finally came out on Aug. 27. The only thing they did was replace the top board of the railing and paint it (not sure if it even matches the paint sample) and replace some floor boards and slap on one (?) coat of stain/seal on top only. The guy came to the door and said they were done for the day. We thought it was strange that they didn't sand the vertical bars of the railing before they stained the other boards. I called Brandon the next morning - he said the job was done. What? "We only did what you contracted us to do." Hello, you're the professional. Shouldn't you explain your estimate better so that the homeowner understands exactly what is going to be done or not done? He said they did explain it to my tenant and if we didn't get the entire story, that was our problem. Their website states that they do a complete walk through of the job at the beginning and the end and don't collect payment until the customer is satisfied. Lies! They didn't do any of it and still had the nerve to send someone over to collect payment the next day. When I called to talk about the poor work ethics, miscommunication, and unfinished (in my opinion) job, I got the runaround from Brandon and my daughter was told via email that they were going to use our situation as a training tool. Very poor customer service from start to finish.
Description of Work: Replaced top rail board of deck, replaced several worn boards on the deck floor, one (?) coat of stain/seal on deck floor

Rating CategoryRating out of 5
quality
3.0
value
1.0
professionalism
1.0
responsiveness
1.0
punctuality
1.0

$850

Response from Deck Doctor Llc
Member allowed her son in law to be her contact between Deck Doctor and herself, because he lives there and she lives out of town. He failed to relay recommendations and options given by Deck Doctor to her for the refinish and repair of the deck.She failed to question her signed work agreement as to the work that was agreed to be done. Contact names, numbers and availablility were given for any questions or concerns prior to work performed. All scope of work was reviewed, discussed and agreed to before work started. We attempted to discuss and address her concerns after the work was completed. She wanted Deck Doctor to do additional work without compensation based on her confusion as to the scope of work. Member became demanding using foul language and attempting to bully us into her demands. Deck Doctor represented itself and the work to be done in a clear and professional manner to her representative, with a face to face meeting, pictures, notes, samples, emails and phone conversations, Deck Doctor has done all it can to help resolve this to a agreeable compromise.

Sariel B.
06/2012
3.0
decks
  + -1 more
I found Deck doctor on google.Called him and he called back shortly after.he came over and gave us an estimate.the day of the work was important to us and he was able to accommodate us.He did a few jobs , he removed the an old deck , added sections to a privacy wall and renewed an older bench on the patio.all of these jobs were done well.HOWEVER he also powered wash the patio deck ,sanded it and stained it.After the work was done it looked great BUT very shortly after we noticed that the color of the wood was fading and the deck looked tired , as if a cloud of dust was on it.We thought it ws dust so we swept the deck a few times and washed it , But the luster and the nice look was gone ! We contacted Dr. Deck and told him about our disappointment.He said he will call us back with dates of when he can come over , but he didn't call back and I had to call him again to set up an appointment.He came over an told us that in his opinion the deck is beautiful.he told us that the deck wood that was used was meant for inside a home and not an out side a home and that is how this deck will look like, he also said something about the moisture under the deck that makes it look like that.that if we wanted a shiny deck , he doesn't provide this service .We were very disappointed to hear that as this was the first time we heard that this is what the deck will look like .Had he communicated with us in the first place , we may have made other decisions and not spent so much money just in sanding and staining.The work we though we were going to get , we did not get and the deck only after 2 months looks old and tired.We then asked him to recommend a product that maybe can clean the wood and he could not recommend a product. We parted very upset we will not recommend him to any one .
Description of Work: I found Deck doctor on google.Called him and he called back shortly after.he came over and gave us an estimate.the day of the work was important to us and he was able to accommodate us.He did a few jobs , he removed the an old deck , added sections to a privacy wall and renewed an older bench on the patio.all of these jobs were done well.HOWEVER he also powered wash the patio deck ,sanded it and stained it.After the work was done it looked great BUT very shortly after we noticed that the color of the wood was fading and the deck looked tired , as if a cloud of dust was on it.We thought it ws dust so we swept the deck a few times and washed it , But the luster and the nice look was gone ! We contacted Dr. Deck and told him about our disappointment.He said he will call us back with dates of when he can come over , but he didn't call back and I had to call him again to set up an appointment.He came over an told us that in his opinion the deck is beautiful.he told us that the deck wood that was used was meant for inside a home and not an out side a home and that is how this deck will look like, he also said something about the moisture under the deck that makes it look like that.that if we wanted a shiny deck , he doesn't provide this service .We were very disappointed to hear that as this was the first time we heard that this is what the deck will look like .Had he communicated with us in the first place , we may have made other decisions and not spent so much money just in sanding and staining.The work we though we were going to get , we did not get and the deck only after 2 months looks old and tired.We then asked him to recommend a product that maybe can clean the wood and he could not recommend a product. We parted very upset we will not recommend him to any one .

Rating CategoryRating out of 5
quality
2.0
value
4.0
professionalism
2.0
responsiveness
4.0
punctuality
5.0

$2,500

Response from Deck Doctor Llc
Member is unhappy with the look of oil based semi transparent stain. He would now prefer the look of a product that gives a shiney reflective apperence like the finish of interior wood floors. Given that preference, a differant product to reflect that finish would have been recommended. The next refinish of his deck can produce exactly what he would like to see. The only option at the time of bringing the disappointment to our attention was a sanding and refinish with a different product. He was not interested in that option. His deck looks beautiful and exactly what a oil, semi transparent stain offers. Deck Doctor attempted to help with the displeasure, were sure in time the Member will have exactly the look on his deck he's looking for.

Julie A.
05/2012
5.0
decks
  + -1 more
We had a great experience with this company. Our sad looking deck now looks like new and we can't wait to enjoy it this summer. They were prompt, professional, did great work, and were true to their estimate. We would definitely use Deck Doctor again. Thanks for the great service!
Description of Work: We had our deck sanded and restained - it was in dire need of a facelift!

Rating CategoryRating out of 5
quality
5.0
value
5.0
professionalism
5.0
responsiveness
5.0
punctuality
5.0

Yes, I recommend this pro
$600

Cindy F.
04/2012
5.0
deck cleaning, decks
  + 0 more
Everything went really smoothly. We would definitely call them again!
Description of Work: We are extremely pleased with the work performed by Deck Doctor. We are planning to sell our home soon, so we were looking for a company who could repair and refinish our deck, making sure everything was up to code. The manager of the company is a former home inspector himself, so he could tell me right away what needed to done. They were a very comfortable company to work with. Their response to our needs was quick and professional, and our deck looks beautiful again!

Rating CategoryRating out of 5
quality
5.0
value
5.0
professionalism
5.0
responsiveness
5.0
punctuality
5.0

Yes, I recommend this pro
$2,775

Joshua M.
03/2012
5.0
decks, fencing
  + 0 more
They were extremely knowledgeable. Their prices were extremely comparable. They did great work. They were quick and easy. The supervisor was very easy to deal with.
Description of Work: I used Deck Doctor Refinishing to refinish my fence and deck.

Rating CategoryRating out of 5
quality
5.0
value
4.0
professionalism
5.0
responsiveness
5.0
punctuality
5.0

Yes, I recommend this pro
$1,300

ELIZABETH G.
06/2005
5.0
deck cleaning
  + -1 more
GREAT PEOPLE! WORKED VERY FAST - VERY FRIENDLY - VERY COMPETITIVE IN PRICING! QUICK RESPONSE TIME - FINISHED RIGHT ON SCHEDULE! WOULD DEFINITELY USE THEM AGAIN!
Description of Work: I HAVE A LARGE DECK ON THE BACK OF MY HOUSE - NEEDED TO BE RE STRIPPED, SANDED AND REFINISHED. WOOD WAS SPLINTERING AND BUBBLING.

Rating CategoryRating out of 5
quality
5.0
value
5.0
professionalism
5.0
responsiveness
5.0
punctuality
5.0

Yes, I recommend this pro
$492

    Contact information

    4370 Kendrick St, Golden, CO 80403

    www.deckdoctorinc.com

    Service hours

    Sunday:
    8:00 AM - 5:00 PM
    Monday:
    8:00 AM - 5:00 PM
    Tuesday:
    8:30 AM - 5:00 PM
    Wednesday:
    8:30 AM - 5:00 PM
    Thursday:
    8:30 AM - 5:00 PM
    Friday:
    8:30 AM - 5:00 PM
    Saturday:
    8:00 AM - 5:00 PM

    Licensing

    State Contractor License Requirements

    All statements concerning insurance, licenses, and bonds are informational only, and are self-reported. Since insurance, licenses and bonds can expire and can be cancelled, homeowners should always check such information for themselves. To find more licensing information for your state, visit our State Contractor License Requirements page.

    *Contact business to see additional licenses.


    Service Categories

    Decks and Porches,
    Deck Maintenance,
    Fencing

    FAQ

    Deck Doctor Llc is currently rated 4.1 overall out of 5.

    Sunday: 8:00 AM - 5:00 PM

    Monday: 8:00 AM - 5:00 PM

    Tuesday: 8:30 AM - 5:00 PM

    Wednesday: 8:30 AM - 5:00 PM

    Thursday: 8:30 AM - 5:00 PM

    Friday: 8:30 AM - 5:00 PM

    Saturday: 8:00 AM - 5:00 PM

    Deck Doctor Llc accepts the following forms of payment: Check,MasterCard,Visa
    Yes, Deck Doctor Llc offers free project estimates.
    No, Deck Doctor Llc does not offer eco-friendly accreditations.
    No, Deck Doctor Llc does not offer a senior discount.
    No, Deck Doctor Llc does not offer emergency services.
    No, Deck Doctor Llc does not offer warranties.
    Deck Doctor Llc offers the following services: Deck sanding, deck or redwood sealant application, power washing, deck repair, fence refinishing & composite work.

    Contact information

    4370 Kendrick St, Golden, CO 80403

    www.deckdoctorinc.com

    Service hours

    Sunday:
    8:00 AM - 5:00 PM
    Monday:
    8:00 AM - 5:00 PM
    Tuesday:
    8:30 AM - 5:00 PM
    Wednesday:
    8:30 AM - 5:00 PM
    Thursday:
    8:30 AM - 5:00 PM
    Friday:
    8:30 AM - 5:00 PM
    Saturday:
    8:00 AM - 5:00 PM